Historically, The Kingdom of the North ceased to exist by 580AD with the foederati in Pedwar rebelling against the locals sometime after 550AD. The tidbits of information that the rules provide is just enough to spark your interest to find out a bit more. So I began to dig.
It seems that the northern area was a kingdom under an Old King Cole (Coel Hen) (Yes the nursery rhyme guy) who was quite possibly a Dux Britanarium or warlord of northern Britain. He lived from approximately 350 to 422AD. Following Celtic tradition, his lands were divided among his sons (Ceneu and Gorbanian) following his death.
The area I am interested in was ruled by Ceneu (circa 382 to 450ish). He allowed Saxon settlement in Deywr (later Deria) in recognition of the Saxon foederati under Octha and Ebissa in Ceneu's campaigns against the Picts and Scotti to the North. Again, the kingdom was divided among his two sons following his death. Gwrast took the western half of the kingdom and Mor took the eastern half with the capital of Ebauc/Eburac/York.
Here is where the tale begins to raise eyebrows. Mor is also called Uther Pendragon in some places. He has a son named Arthuis as well. Mor was born around 420AD and had two sons. Apparently he died close to 470AD. Unlike others, it appears that only one son inherited - Arthuis.
Arthuris was born around 455AD. He had two sons as well. Eliffer Gosgorddfawr (or many spears) and Pabo Post Prydein. Eliffer lived around 473AD to 560AD. He apparently ruled the kingdom on his own.
Of Eliffer's sons, they co-ruled the kingdom. Peredur Arueu Dur/Peredur Paladr Hir and Gwrgi fought a number of battles. Notably, they fought in the Battle of Arfderydd/Arthuret against the King of Caer-Wenddoleu named Gwenddoleu ap Ceidio. They then formed an alliance with two other kingdoms (Strathclyde and North Pennines) and fought Gwenddoleu again and killed him in 573AD. This is probably the battle of Arthuret but some sources seemed to treat it as a separate battle. Gwenddoleu ap Ceidio's bard, Myrddin Wyllt (or Merlin) goes mad after witnessing the defeat.
Eliffer's sons begin to have a spot of bad luck. Seven years after their victory, they go to war with the Saxons of Bernicia/Bernaccia. Both are killed by King Adda at the battle of Caer-Greu. Within a year, the Derians under King Aelle (remember the foederati that King Ceneu gave land to?) rise up and sieze the kingdom and Ebrauc (York). The children of Gwrgi and Peredur Arueu Dur flee for their lives.
This actually gives a good spread of possibilities for a campaign. The kingdom repeatedly fights not only Saxons, but Picts and other Britons as well. This should provide for a dynamic campaign to run. The more I read up on the period, the more fun I think I will have.